The present invention relates to generally a parison molding machine and more particularly a parison length controller therefore for automatically controlling the wall thickness of a parison being molded, thereby maintaining it to a predetermined length.
The parison molding machine or the extrusion-blowing machine vertically extrudes semi-molten plastic material through the die orifice between a circular die and a mandrel into a mold in the form of a hollow cylinder, thereby forming a parison.
The wall thickness of a parison is dependent upon the degree of opening of the die orifice which in turn is dependent upon the position of the mandrel with respect to the die. That is, depending upon the configurations of the dies used, a parison controller controls the degree of opening of the die orifice. As disclosed in U.S. Pat. No. 4,011,493, granted Mar. 8, 1977 to Hisahiko Fukase and Minoru Yoneda who are also the inventors of the present invention, the parison controller automatically moves the mandrel vertically depending upon the configurations of the die used so as to control the degree of opening of the die orifice, thereby controlling the wall thickness distribution of the parison which is being extruded.
Since a predetermined quantity of plastic material is extruded in every extrusion cycle, parisons having a predetermined length are formed as far as the uniform wall thickness distribution is maintained. However in practice the length of parisons varies because of the variations in temperature, density and viscosity of plastic material and in configuration of the dies used. So far the parisons having a length in excess of a predetermined length have been cut to a predetermined length, but those short of a predetermined length have been rejected so that the parison production efficiency is low and a considerably large amount of materials is wasted for nothing.